Fix upgrade and uninstall functions to pick up $ZSH value

lib/functions.zsh

@@ -3,11 +3,11 @@ function zsh_stats() {
 }
 
 function uninstall_oh_my_zsh() {
-  /bin/sh $ZSH/tools/uninstall.sh
+  /usr/bin/env ZSH=$ZSH /bin/sh $ZSH/tools/uninstall.sh
 }
 
 function upgrade_oh_my_zsh() {
-  /bin/sh $ZSH/tools/upgrade.sh
+  /usr/bin/env ZSH=$ZSH /bin/sh $ZSH/tools/upgrade.sh
 }
 
 function take() {